A tap-changer monitoring system incorporating optical sensors

The National Grid Company (NGC) owns and operates approximately 600 transformers of various ratings up to 1000 MVA. These transformers are very reliable, and have extremely long lives. They have no moving parts, and the main degradation mechanism is deterioration of the paper insulation, which is a very long-term process. The vast majority of transformers at ratings up to 240 MVA have on-load tap-changers to vary their turns ratio, and in marked contrast to the windings, these are complex mechanical switches with many moving parts. It is not surprising that they are less reliable than the windings, and a significant proportion of the serious transformer faults can be attributed to problems originating in the tap-changers. A monitoring system which could detect incipient failures and act to prevent them would thus be of benefit, providing it could be produced at reasonable cost. In addition, by monitoring the performance of the tap-changer continuously, it may be possible to move away from maintenance done at fixed time intervals, and towards a condition-based programme, which could result in significant savings. The authors describe a chromatic modulation technique monitoring system which uses a fibre optic sensor.